The invention relates to an optical transducer comprising optical detection means including a first, a second, a third and a fourth optical detector, such that a first imaginary line from the third optical detector to the first optical detector crosses a second imaginary line from the second optical detector to the fourth optical detector which optical detectors generate detection signals in response to an intensity of radiation incident thereon, the optical transducer further comprising signal combination means for generating a combination signal which is indicative for a sum of electrical detection signals.
The invention further relates to a recording/playback device comprising the transducer.
A transducer as described in the opening paragraph is used in commercially available CD-players. In the known transducer, the combination means generate a combination signal which is indicative for the sum of the detection signals of the first, the second, the third and the fourth detector. This signal as well as four signals indicative for the detection signals of each of the detectors are provided via a flexible cable to a signal processing unit. The combination signal serves as an input signal for a data retrieval unit comprising bit detection means, channel decoding means and error correction decoding means. The four signals indicative for the detection signals are used to calculate a DPD tracking signal.
It is a problem that the known optical transducer is not suitable for high data speeds. E.g. for reading DVD at 20X it would be necessary to transport 5 signals having a frequency of 180 MHz via the flexible cable. Interference could be reduced by transporting each of the signals in the form of two output signals being mutually in counterfase. However this would require a relatively large number of conductors, only for these signals.
It is a purpose of the invention to provide a transducer according to the opening paragraph, which is suitable for use at high data-speeds, while requiring a relatively small number of output signals. For this purpose the optical transducer according to the invention is characterized in that the combination signal is indicative for the sum of the electrical detection signals of the first and the third optical detector the signal combination means further being comprised for generating a first, a second and a third additional combination signal, the first additional combination signal also being indicative for the sum of the electrical detection signals of the first and the third optical detector, the first additional combination signal and the combination signal having a mutually opposite phase, the second and the third additional combination signal each being indicative for the sum of the electrical detection signals of the second and the fourth optical detector, the second and a third additional combination signal having a mutually opposite phase.
In the transducer according to the invention only four conductors are necessary to transport the two detection signals. Despite this, the two detection signals can be used both for track detection by means of the DPD-method and for data retrieval. The individual detection signals, which may be used for generating a focus error signal, may be transported at a relatively low speed, e.g. at a third or a fourth of the speed of the detection signals. These signals therefore may be transported each by a single conductor.
Preferably the optical detection means further comprise a fifth optical detector which is arranged in a direction from the first and the fourth detector to the second and the third detector of an imaginary line beside the second and the third optical detector and a sixth optical detector arranged in said direction beside the fifth optical detector, the detection means further comprising a seventh optical detector arranged in a direction opposite the first direction beside the first and the fourth optical detector, as well as an eighth optical detector arranged in a direction opposite the first direction beside the seventh optical detector.
The additional fifth until the fourth detector enable other tracking methods, e.g. the three spots central aperture or the three spots push pull tracking method.
A preferred embodiment of the optical transducer according to the invention is characterized by input means for receiving one or more mode selection signals, mode selection means responsive to the one or more selection signals, which mode selection means enable a plurality of operational modes, in which at least one output signal is a selectable function of one or more of the electrical signals generated by the optical detectors. In this embodiment the optical transducer can be adapted for different applications, e.g. CD, CDR/RW or DVD while the number of connections can be limited.
In an implementation of this embodiment the optical transducer is characterized by a first until an eighth output signal, the optical transducer having an operational mode (M1), wherein the first and the second output signal each are representative for the sum of the electrical signals of the first until the fourth detector, the first and the second output signal being of opposite phase, the third output signal being representative for the sum of the electrical signals of the seventh and the eighth detector, the fourth output signal being representative for the sum of the electrical signals of the fifth and the sixth detector, the fifth until the eighth output signal respectively being representative of the electrical signals of the first until the fourth detector.
In this operational mode the optical transducer is in particular suitable for reading CD. The first and the second output signal can serve as input signals for a data retrieval unit. The third and the fourth output signals can serve as satellite signals for a three spot central aperture detection method.
A further implementation of the above described preferred embodiment is characterized by a first until an eighth output signal, the optical transducer having an operational mode (M2), wherein the first and the second output signal each are representative for the sum of the electrical signals of the first until the fourth detector, the first and the second output signal being of opposite phase, the third output signal being representative for the sum of the electrical signals of the fifth and the eighth detector, the fourth output signal being representative for the sum of the electrical signals of the sixth and the seventh detector, the fifth until the eighth output signal respectively being representative of the electrical signals of the first until the fourth detector.
In said operational mode the optical transducer is in particular suitable for recording and playback of CDR/RW. The first and the second output signals are suitable for data retrieval, while the third and the fourth output signals can be used to derive a tracking error signal by means of the three spots push-pull method.
A still further implementation of the optical transducer is characterized by a nignth output signal, the optical transducer having an operational mode (M0) wherein the first output signal is representative for the sum of the electrical signals of the first until the fourth detector, the second until the fourth output signal respectively are representative for the electrical signals of the eighth, the seventh and the fifth detector, the fifth until the eighth output signal respectively being representative of the electrical signals of the first until the fourth detector, the ninth output signal being representative of the electrical signal of the sixth optical detector.
An interface with this set of output signals is a defacto standard. In this operational mode the optical transducer can be used in conventional products.
These and other aspects of the invention are described in more detail with reference to the drawings. Therein: